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REPTILIOMORPHA | `--AMNIOTA |--Casineria |--Westlothiana `--Crown Amniota |--Sauropsida (= Reptilia) | |--ANAPSIDA | `--EUREPTILIA `--SYNAPSIDA
Reptiles - those creeping, crawling, slithering, scaley, animals, dragons and dinosaurs, creatures of myth and folklore. Who hasn't had their imagination stirred by these creatures and their chthonic, typhonic, consciousness?
Yet in order to understand reptiles and their place in the evolutionary history of life on Earth, we need to get rid of a few myths and misconceptions
A lot of what the common person knows or thinks about reptiles is determined by cultural prejudice. The whole idea of the reptile, especially derived squamata, as the archetypal reptile, as a cold-blooded, terrifying, evil and sinister creature, has its roots in teh Judae-Christian creation myth (bottom left), although its hould be pointed out that there are also more subtle and psychologically nuanced readings than the usual literalism. But it's just a small leap of unscientific imagination from the serpent in the Garden to Extraterrestrial shapeshifter (below center). The biblical phobia of creeping animals probably also ties in with the new paradigm meme of the reptilian brain as the most archaic, atavaistic, and savage stratum of brain evolution (below right)
Negative, Western Judaeo-Christian inspired, images of reptiles:
Not surprisingly, all of these myths and prejudices are incorrect. Rather than being "cold blooded", reptiles are poikilotherms, in that their body temperature varies according to environmental conditions or physiological activity (and in the tropics or even temeprate regions after being warmed by the sun or a period of activity they would be "warm blooded"), in contrast to homeotherms such as some fish, dinosaurs, birds, and mammals which maintain thermal homeostasis (feedback and equilibrium. In premodern worldviews, worms and snakes and other limbless crawling creatures are easily conflated, similar to the biblical confusion of birds and bats, The whole New Age reptilioid conspiracy theory (as well as ufo reptilians in general) can be memetically derived from (as well as chronologically predated by) the miniseries and television series "V"; serpents. The triune brain hypothesis, with its linear succession of reptilian old brain, paleomammalian limbic system, and neommalian neocortex, is no longer held to be a valid model of the evolution of the forebrain, classical reptiles rather than being savage and instinctual often display complex behaviour, intelligence, and even care of the young, birds (which are essentially modified (dinosaurian) "reptiles") have sophisticated neocortical learning fuctions, and some fish "limbic" style parental care ).
Interestingly, in other religions, such as Hinduism, Hermeticism, and some forms of Gnosticism, which are based more in contemplative spirituality and meditation rather than authoritarian legalism, snakes have a much better reputation, being considered symbols of wisdom. We find this more positive association even in the current fantasy genre, where the dragon generally has a high status.
Positive images of reptiles:
Photo by Prioryman, Wikipedia, GNU Free Documentation/Creative Commons Attribution
Photo by DoktorMax, Wikipedia, public domain
Photo by Carlos adanero, Wikipedia, public domain
As someone who loves reptiles (and other creatures) the present author cannot help but feel more attraction for these positive images, and a strong antipathy for the negative, religious and conspiracy theory memes.
But of reptiles themselves? Freed of human anthropocentrism and psychological projections both positive and negative, they are an assortment of animals living their own lives and evolving in their own direction.
Their story is indeed a fascinating one, but whether reptiles even constitute a valid taxon is now a disputed in biology. Originally they were given the rank of class in the Linnaean system (although originally grouped by Linnaeus with the amphibians in his Systema Naturae). The Linnaean system howeveris a static classification, not a dynamic picture of the tree of the evolution of life on Earth. Our understanding of the evolutionary history of reptiles and their era they ruled the Earth seems to have passed through three broad stages.
In the 19th century, ancient saurians caught the popular imagination with the great paleontological discoveries of the Victorian Age. Hence images of antediluvial monsters, primordial creatures of ancient swamps. This soon settled down to more serious science, although it was a while before dinosaurs were no longer sprawling overgrown lizards.
By the late nineteenth and early twentienth century, Darwin's revolution had well and truly shaped scientific thinking, replacing early 19th century Owenist archetypes and Cuverian sequential creationisms of the early 19th century with a synthesis of Darwinian natural selection, Mendelian inheritence, population genetics, and paleontology and a sense of evolution through deep time. This is represented by the Evolutionary Systematics. Here the books of Romer and, on a less technical level, Colbert, informed a generation of budding paleo enthusiasts
The diagram at the left can be compared with the similar diagram regarding the evolution of amphibians. Both have ancestral groups (ichthyostedids, cotylosaurs, thecodonts) from which radiate a whole slew of descendants; both have familiar and easily defined (in terms of recent species) categories such as classes Amphibia and Reptilia.
Among the advantages of evolutionary systematics is the emphasis on ancestral groups and clear and easily identifiable categories (we all know what a reptile is, or a bird or a mammal). The disadvantages are a lack of detail when it comes to the order of branching of the phylogenetic tree of life (instead there is simply a sort of star pattern radiating of many different groups from a general ancestor) and that, whilst familiar representatives of a group are recognisable (we easily associate a scaly crawling crocodile with "reptile", but what about a furry, warm-blooded pterosaur). Because of this, the characteristics that define the group as a whole can be generic or ambigious, and often both primitive ancestral and highly specialised or derived types often cannot be so easily identified. The phylogenetic-based cladistic system was an attempt to introduce precision into this whole arrangement. It does this by rejecting all paraphyletic groups. Here the old Class Reptilia becomes the paraphyletic taxon par excellence. This is because reptiles are defined only by shared primitive characteristics, essentially all amniotes that are not mammals or birds are reptiles; a cladistically meaningless statement (whether it is phylogenetically meaningless depends on your definition of phylogeny - evolutionary systamists and cladists use the same word in very different ways!). Instead, reptilia is replaced by a number of other nested clades, as shown in the diagram on the right. Here, the green shaded area represents the Linnaean taxonomic category "reptiles" (or Class Reptilia). This includes primitive synapsida - the pelycosaurs and therapsids (shown by the shading over the bottom half of the line leading to Synapsida; this is not technically the correct way to present a cladogram (the diagonal here would have to be divided into further branching points), but is still useful as a simple diagram) - but not advanced synapsids (mammals). It also inckudes turtles and tortoises (testudines), lizards and snakes and their relatives (lepidosaurs), and crocodiles and many extinct related types, but not birds (class Aves), despite their being closely related to crocs, as the diagrams. Obviously, the two systems are totally incompatible, which doesn't mean that one is wrong and the other right. Simply that they are two different ways of making sense of the natural, biological world. Of these two systems, cladstics has been almost universally adopted by vertebrate paleontologists, because it provides a standardised methodology for testing phylogenetic hypotheses (i.e. proposals about phylogeny, not the actual deep time historical fact of phylogenetic relationship). That doesn't mean that evolutionary systematics is automatically wrong, just its a different methodology; all these methodologies are useful each in their own way.
For this reason, we retain use reptile here in both the colloquial, the Linnaean, and the evolutionary systematic, sense of four-legged or secondarily limbless, ectothermic (heat from external environment) or poikilothermic (body temperature varies widely) animals (more properly vertebrates) (apart from dinosaurs and pterosaurs which were it seems homeotherms) which lay shelled eggs (except for some snakes and Mesozoic marine reptiles that give or gave birth to live young), with a skin covered in scales and/or scutes (or secondarily smooth as with some of the afore-mentioned marine reptiles). If this sounds excessively vague, you can understand why many have made the switch to cladistics. On the other hand, it still serves, like "protist, "plant", "invertebrate", or "fish", as a useful generic term that is a lot easier to say and use then "any amniote that is not a bird or mammal". The same ease of use applies to other paraphyletic terms, which is why we have retained them, rather than resort to tedious phrases such as, to give just one example, "non-crocodylomorph, non-dinosaur, non-pterosaur stem-group archosaur/archosauriform" when the old 19th century term "thecodont" would in that instance serve quite nicely.
As for the cladistic (as opposed to Linnaean) definition of Reptilia, which is the most recent common ancestor of all recent reptiles and all their descendents (see technical definition below.), this term is now redundant, since it depended on a particular group of prehistoric reptiles, called mesosaurs, diverging from the phylogenetic tree imemdiately after synapsids but before all other groups. Since this is now pretty much agreed not to be the case, clade Reptilia becomes synonymous with clade Sauropsida, assuming teh traditional hypothesis of turtles (testudines, chelonia) as anapsida holds. However, if turtles and their ancestors turn out to be diapsids (and more specifically crown group diapsids), that means only diverged much later. In thios case, Reptilia becomes a sub-clade of Sauropsida. All of which shows what a world of controversy something as unassuming as phylogeny can be. MAK111121,
Image credits: Dragon, Dani 7C3, Wikipedia; Cladogram Petter B°ckman, Wikipedia, Evolution of reptiles from Schaffer/Bonine/Ferriere
 There was a time when one of us (MAK) was quite impressed with neuroscientist Paul MacLean's model, which was a standard part of the "New Paradigm" worldview of the late 70s and 80s and popularised by Carl Sagan (Dragon's of Eden), and also referred to by other authors such as Arthur Koestler and Erich Jantsch. But, as is so often the case, science moves on. See for example Bruce & Neary 1995, Lanuza et al 1998 (references via Neuro-Dojo), Striedter 2005, and Patton 2008 (refrences via Wikipedia)
Reptilia: LCA Turtles and birds (link).
Range: from the Late Carboniferous.
Phylogeny: evolutionary systematic definition: see stem amniota
cladistic definition (1): Originally defined as Sauropsida: Mesosauridae + *: Anapsida + Eureptilia. Definition no longer holds, now that mesosaurs are almost universally considered anapsids,
cladistic definition (2): LCA of all living reptiles. If turtles are anapsids, Reptilia is synonymous with Sauropsida. If they are diapsids, Reptilia is synonymous with Sauria. MAK111121
Characters: Tabular small or absent; large posttemporal fenestra; suborbital foramen (small hole near the lateral edge of palate, between the pterygoid, palatine, and ectopterygoid or jugal, when ectopterygoid absent); supraoccipital plate narrow.
Links: Class Reptilia; THE EMBL REPTILE DATABASE; Reptilia - Suite101.com; Introduction to the Diapsids; Reptilia (Reptiles); Nuova pagina 1; BIOSIS | Resource Guide | Reptilia; Class Reptilia; Reptiles; Phylogeny and Classification of Amniotes; Photogallery: Reptilia \ Amphibia. ATW020623.
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